Automatic selectivity control in radio receivers



Oct. 1947. J VERBEEK 7 2,429,734

AUTOMATIC'SEDECTIVITY CONTROL IN RADIO RECEIVERS Original Filed March11, 1936 INVENTOR fang; yerzwk 7% w :AT'TORNEY Patented Oct. 28, 1947 sPATENT OFFICE AUTOMATIC SELECTIVITY CONTROL IN RADIO RECEIVERS Henri P.J. Verbeek, Eindhoven, Netherlands, assignor, by mesne assignments, toHartford National Bank and Trust Company, Hartford,

001111;, as trustee Original application March 11, 1936, Serial No.

68,172. Divided and-this application November 13, 1941, Serial No.418,930.

April 13, 1935 In' Germany 2 Claims. (01. 250-20) This invention relatesto a circuit arrangement for use in radio receiving sets and for thepurpose of automatically controlling the width of a frequency bandtransmitted by a tuned high or intermediate frequency amplifier. Thisapplication is a division of my application Serial No. 68,172,

filed March 11, 1936.

For the purpose of obtaining an optimum reproduction by radio receivingsets of speech or music transmitted by a broadcasting stationit isdesirable that on either side of the carrier wave a frequency band ofabout kilocycles should be evenly amplified by the tuned high and/orintermediate frequency amplifier of the set. Since, however, with thepresent day distribution of wave lengths of the broadcasting, stationsthe carrier wave frequencies are spaced about 9 kilocycles apart, theradio receiving sets are so arranged that they only transmit at theexpense of the quality of reproduction a frequency band of about 4kilocycles on either side of the carrier Wave frequency.

Now, all transmitters are not received with identical intensity so thatwhen receiving from transmitters the intensity of which is substantiallyhigher than that of the neighboring transmitter it is possible for theband width of the high and/or intermediate frequency amplifier to beincreased without the risk of troublesome interference phenomenaoccurring. It has therefore previously been proposed to provide radioreceiving sets with a band width adjustment which either is operated byhand or is controlled automatically in accordance with the intensity ofthe signal received. Such an automatic band width adjustment isobtained, for example, by the automatic volume control which controlsthe amplification of the set in accordance with the amplitude of thecarrier wave received acting simultaneously on the coupling of the highand/or intermediate frequency band filter, for example by thecoupllng ofthe coils of a tuned band filter comprising two inductively coupledoscillatory circuits being increased with increasing signal intensity sothat the band width let through increases. These well known automaticband width controls have however, the disadvantage that the band widthlet through only depends on the intensity of the signal received and iswholly independent of the intensity of the neighboring transmitter. Thisleads to interference disturbances when the neighboring transmitters arereceived with sufficient intensity. According to the invention, thisdisadvantage is obviated by automatic control of the band width inaccordance with the carrier wave amplitude of the neighboringtransmitter.

In order that the invention may be clearly understood and readilycarried into effect two circuit arrangements in accordance therewithwill now be described more fully with reference to the accompanyingdrawing. Figures 1 and 2 disclose two embodiments of the invention, andonly so much of the receiving circuit as shown for a properunderstanding thereof.

Referring to Figure 1, which illustrates the essential circuitarrangement, two amplifier valves I and 2 of an intermediate frequencyamplifier comprising inductively coupled band filters 3 and 4 areillustrated. The band width may be adjusted by varying the couplingbetween the coils of the band filters in that they are pushed more orless towards each other. This control is effected by an electromagnet 5and is shown schematically by the dotted line. The latter is controlledin the circuit arrangement of Figure 1 by the anode current of anamplifier valve 6, the grid voltage of which depends on the voltage dropacross two series-connected resistances I and 8. The load resistances Iand 8 are inserted respectively in the direct current circuits of twodiode rectifiers 9 and Ill. The diode rectifiers 9 and ID are providedwith the tuned input circuits I I and I2 respectively, having oneterminal connected to the anode and the other terminal thereof connectedto the cathode through the respective resistances 8 and I. The negativeend of resistance 8 is connected to the control grid of amplifier 6, thenegative end of resistance I is connected to the cathode of rectifier 9,and the cathode of rectifier I0 is connected to the cathode of amplifier6. The latter connections are such that load resistances I and 8 arealways in series between cathode and control grid of amplifier 6, thevoltage developed across either resistance determining the biasimpressed upon the amplifier grid 6. As shown the tuned circuits I I andI2 are coupled, for example, to the band filter 3 of the interme-' diatefrequency amplifier. The circuit I I is tuned to a frequency exceedingby 9 kilocycles the intermediate frequency and the circuit I2 is tunedto a frequency 9 kilocycles below the intermediate frequency. Thearrangement is adjusted in such manner that when a transmitter is notreceived, the coupling between the coils of the band filters 3 and 4 isa maximum and these band filters allow the passage of a frequency bandof about 9 kilocycles on either side of the intermediate frequency. Thiscondition is still unvaried on receipt of a transmitter having amaterially higher carrier Wave amplitude than the neighboringtransmitters since in this case in the intermediate frequency amplifier,when the neighboring transmitters diifer by 9 kilocycles from thetransmitter received, the amplitude of the lateral band frequency whichdiffers by about 9 kilocycles from the intermediate frequency proper isbut small. In this case the alternating voltage supplied to therectifiers 9 and I and the direct currents passing through theserectifiers are also small and thus a substantial variation of thecurrent passing through the electro-mag-net is not brought about so thatthe adjustment of the band width is still unaltered.

If, however, a neighboring transmitter has a considerable intensity sothat an intermediate frequency alternating voltage which differs fromthe intermediate frequency proper by about 9 kilocycles and ha asufficiently high amplitude occurs, the grid voltage of the tube 6becomes more negative due to the direct current passing through therectifiers 9 and I0 so that the anode current of the tube 6 which passesthrough the electro-magnet decreases. The coupling between the coils ofthe band filters 3 and 4 is thus decreased and the width of thefrequency band transmitted by the said band filters decreases unti1 agiven position of equilibrium in which the interference noise of 9kilocycles is no longer troublesome ensues.

Now, the various broadcasting stations are not spaced apart exactly 9kilocycles. For this reason, it may be advantageous to tune the circuitsII and [2 to frequencie differing from the intermediate frequency, forexample, by from '7 to 8 kilocycles instead of by 9 kilocycles.

In the circuit arrangement shown in Figure 1 the arrangement for theautomatic band width adjustment is controlled by the intermediatefrequency amplifier. In the circuit arrangement shown in Figure 2 thiscontrol is effected from the low frequency part of a receiving set. Inthis circuit arrangement an intermediate frequency band filter I3, theband width of which is automatically adjusted by an electromagnet I4, iscoupled to the second detector l5 of the set. The low frequencyalternating voltage occurring across the resistance 16 controls the lowfrequency amplifier of the set via connecting conthe anode circuit ofwhich includes the relay M. The filter 20 is so proportioned as totransmit only frequencies exceeding, for example, 6 kilocycles. Theoperation of thi circuit arrangement is identical with that of thecircuit arrangement shown in Figure 1.

It is of course obvious that the circuit arrangement according to theinvention is also applicable to high frequency amplifiers.

The invention is not limited to the described regulation of the width ofthe transmitted frequency band by control of the coupling factor of theband filter but also applicable to any other well known control of theband width.

What I claim is:

1. In a, modulated-carrier signal receiver, a band-pass selector forselecting a. desired signal comprising a carrier frequency and a band ofmodulation frequencies, means for deriving from said desired signal andan undesired signal on a carrier frequency near the desired signalcarrier frequency a control signal comprising beat notes of saidsignals, and means responsive to the amplitude of said control signalfor adjusting the Width of the band of frequencies passed by saidselector inversely in accordance with said amplitude of said controlsignal.

'2. In a modulated-carrier signal receiver, a band-pass selector forselecting a desired signal comprising a carrier frequency and a band ofmodulation frequencies, means for deriving from said desired signal andan undesired signal on a carrier frequency near the desired signalcarrier frequency a control signal comprising beat notes of saidsignals, and means responsive to the amplitude of said control signalfor adjusting the width of the band of frequencies passed by saidselector inversely in accordance with said amplitude of said controlsignal, said deriving means comprising a high pass filter adapted topass frequencies over 6 kilocycles.

HENRI P. J. VERBEEK.

REFERENCES CITED The following references are of record in the file ofthis patent: 1

UNITED STATES PATENTS Number I Name Date 2,255,645 Bligh et a1 Sept. 9,1941 2,017,523 Beers Oct. 15, 1935

